Automatic transmissions are designed to adapt the power of an engine to meet automatically varying road conditions. A typical automatic transmission is provided with electromagnetic operating means for causing the automatic transmission to shift up or down according to a predetermined shift control schedule depending upon road speed, throttle position and engine load. In an attempt to prevent the automatic transmission from hunting upon shifting, it is popular to design the automatic transmission to shift in accordance with a specifically designed up- and down-shift control pattern or schedule in which a hysteresis is provided between up- and down-shift control lines. However, the automated transmission thus designed may still tend to hunt, in particular responsive to a slight opening or closing of an accelerator pedal when the vehicle is traveling at a constant speed. To avoid such hunting upon shifting, for example upshifting, it was thought that the upshift of the automatic transmission should be delayed for a predetermined or preselected period of time when the vehicle is traveling in a low speed range, where the tendency for hunting is more noticeable.
In automotive vehicles equipped with automatic transmissions, it is important to make efficient use of the braking effect of the engine without manually changing gear to slow down the vehicle. To obtain high engine braking efficiency, it is sufficient to operate the vehicle in a low gear speed range. In view of this, it was thought that an automatic transmission should be controlled to shift itself in accordance with an automatic shift control pattern or schedule in which a lower part of a particular gear range extends to a specified vehicle speed in the high gear range one-step higher than the gear range. Such an automatic shift control schedule is known from Japanese Unexamined Patent Publication No. 52-20630 entitled "Automatic Transmission" laid open June 4, 1977.
A drawback caused by delaying the shift control signal for a predetermined period of time, however, is that the automatic transmission still tends to hunt, upon deceleration, when the throttle valve nears or is in its idle position. Thus, when the vehicle slows down or is decelerated in such a way as to change the throttle valve from a certain throttle opening to its idle position, it often happens that the automatic transmission shifts transitionally up from a lower gear range into a higher gear range and eventually down again into the lower gear range. In such a case, a downshift control signal is delayed for a time until after the output of an upshift control signal. This leads not only to undesirable hunting of the automatic transmission upon shifting but also to an interim traveling of the vehicle in a high gear range, resulting in decline in braking effect of the engine. This decline of braking effect can be significant in an automatic transmission of the type that is controlled in accordance with a shift control pattern as described in the above publication.